I. Field of the Invention
The invention relates to an axial flow fan of the type comprising an impeller bearing vanes and arranged in a duct surrounded by a wall.
II. Description of the Prior Art
In the interior of their housings axial flow fans possess a vaned impeller which is rotated by a motor. The rotating impeller pumps a gaseous fluid, more particularly air, in the axial direction from the input end to the output end of the housing. The rate of flow per unit time, known as the volumetric flow V, depends upon, among other factors, on the speed of rotation of the impeller.
The operational characteristics of known axial flow fans become irregular with increased throttling of the volumetric flow. When a certain machine-dependent minimum value is reached, the flow starts to stall in the fan. As a consequence there is a rotating detached flow referred to as rotating stall. The separation of the flow along the vane contour, which commences when there is a considerable decrease in the flow velocity, does not occur simultaneously with all vanes. Initially a sort of separation zone comes into being on the upstream vane side. The resulting dead space is characterized by intensive internal vortices, which result in a considerable loss in energy. Blockage of the passage resulting from the separation causes the flow to be deflected and when a certain critical volumetric flow is reached, the so-called stalling point, individual channels suddenly become blocked. Because this affects the flow towards adjacent vanes, the intervane channels in which the flow is impeded cause the flow to relatively move around the impeller in a direction opposite to the actual direction of rotation. As a result, a heavily pulsating flow is created whereby smooth operation in this instable range is impossible. In extreme cases, the high flexural and alternating flexural forces lead to damage to the impeller and of components operated in conjunction with the fan. In the respective partial load range, the performance curve of the axial flow fan is characterized by a sudden loss in pressure. The operation of the fan in the critical range can be extremely noisy.
Because commencement of the instable operating range is typically only a small distance from the optimum operating point, prior known designs to correct these deficiencies had to be particularly thorough and therefore expensive.
The German patent publication 3,322,295 A has proposed a solution to this problem in which there is an additional annular vaned duct which is to receive the separated flow and the momentum thereof and to cause stabilization. However such a design may only be readily applied to a limited range of conceivable impeller geometries. It is extremely difficult to optimize the vane arrangement in the annular duct, which in any case leads to a substantial increase in price of the fan.